This invention relates, in general, to data transmissions and, in particular, to a system and method for transmitting data to the surface relative to a downhole parameter, such as pressure, temperature, density or position, measured by an instrument positioned downhole.
Well completion and well production require a great quantity of information relating to downhole parameters that characterize various physical quantities. During well completion, as the well is brought to productive status, well logging is utilized as a technique for providing information regarding downhole parameters of the particular earth formation being drilled. Such information typically includes characteristics of the formations traversed by the wellbore, in addition to data relating to the size and configuration of the borehole itself. The collection of downhole parameters during well completion, which is commonly referred to as logging, may be performed by several methods including wireline logging wherein one or more probes having sensors are lowered into the wellbore after some or all of the well has been drilled.
During production as formation fluids are brought to the surface, various downhole parameters are measured and recorded. For example, in order to selectively permit and prevent fluid flow into the production tubing string, it is common practice to install one or more sliding sleeve type flow control devices within the tubing string. A sensor is often positioned proximate to the sleeve to monitor the position of the sleeve. By way of another example, during production, sensors are often positioned within the production intervals of the wellbore to monitor attributes, such as pressure, temperature and density of the formation fluids.
Whether the downhole parameters are measured and recorded during completion or production, the measurements may be processed by a computer and stored in computer memory or other media. The processed measurements may be utilized to create a chart or plot which a formation evaluation professional or driller analyzes in order to gain insight into the well. In general, the evaluation of a formation may be improved by increasing the number of downhole measurements. Balancing the logistical and economical constraints of formation evaluation with the need to increase the amount of formation parameters collected has lead to the utilization of frequency-division multiplexing or FDM which assigns a different frequency band to each sensor so that multiple sensors can share a common communications medium. FDM increases the number of measurements which can be made downhole while minimizing the infrastructure required to transmit these measurements.
FDM is not without limitations, however. Equipment utilizing FDM requires particular downhole and surface components. For example, tone decoders and tuning components are required to properly alter the frequency of the signals and to decode the data carried therein. Additionally, equipment utilizing FDM requires an initial frequency calibration, which has proved to be costly.
Accordingly, a need exists for a system and method for transmitting downhole data to the surface. Further, a need exists for such a system and method to employ a minimum number of components. Moreover, a need exists for such a system and method that are able to carry the signals of multiple sensors over a minimum number of communication medium.